The present invention relates to a polypropylene resin composition, a T die film made of the same, and a method of producing a T die film, and, more specifically, a thermal molded body made of a polypropylene resin composition and a substrate made of a thermoplastic resin.
Films made of a polypropylene resin containing structural units derived from propylene as the main structural unit are excellent in optical properties, mechanical properties and wrapping suitability. Therefore, they are widely used in the wrapping fields such as for food wrapping, fiber wrapping and the like. However, when a T die film used in the fields such as a retort pouch and the like, is produced using a polypropylene-based resin having a narrow molecular weight distribution, there sometimes occurs a problem that the surface of the film is roughened and transparency may be lost.
Further, recently, T die film produced at high speed by a large scale film forming machine is sometimes not satisfactory in transparency, appearance and tearing strength along the MD (machine direction), since film cooling efficiency of the machine is deteriorated and orientation along the MD (longitudinal direction in film formation) increases.
For example, a film obtained by processing at high speed a polypropylene-based resin having a relatively wide molecular weight distribution and having a larger die swell ratio may exhibit unsatisfactory tearing strength along the MD direction, though the extent of insufficiency in transparency and appearance is small.
On the other hand, a film obtained by processing at high speed a polypropylene-based resin having a relatively narrower molecular weight distribution and having a smaller die swell ratio may experience deterioration in transparency, or touch roll marks on the surface of the film which occur when the film is not sufficiently cooled and crystal solidification is insufficient, and the like.
Furthermore, when the resulting film, sheet and the like are going to be subjected to secondary molding process such as thermoformed article and the like, the temperature range in which such sheet is thermoformed is narrow and the processability of thermoforming is poor. Therefore, it is difficult to obtain a laminated molding article which has a body obtained by thermally molding, has excellent production suitability and high scratch resistance.
An object of the present invention is to provide a polypropylene resin composition which avoids the above-mentioned failure in appearance caused by roughening the surface of a film even if a polypropylene-based resin having narrow molecular weight distribution, for example, is used. A T die film obtained by the polypropylene resin composition, and a method of producing the T die film are also provided. Another object of the present invention is to provide a polypropylene resin composition which can provide a film excellent in balance between transparency, appearance and tearing strength along the MD direction even when a T die film is, particularly, produced at high speed, a T die film obtained by the polypropylene resin composition, and a method of producing the T die film.
A further object of the present invention is to provide a laminated molding article which is a thermoformed molding article having a wide temperature range of thermoforming and excellent molding property, and which is excellent in production suitability and has high scratch resistance.
The foregoing objects and other objectives are achieved by the present invention which provides
a polypropylene resin composition comprising 100 parts by weight of a polypropylene resin (A) having a die swell ratio of less than about 1.70 and about 0.01 to about 45 parts by weight of a polypropylene resin (B) having a die swell ratio of about 1.80 or more,
a T die film comprising polypropylene wherein the film has a molecular weight distribution of about 4.2 or less and a die swell ratio of about 1.28 or more, and
a method of producing a T die film comprising the steps of melt-kneading polypropylene by an extruder, extruding the melt-kneaded product through a die lip, and cooling and solidifying the extruded material by a cooling roll to form a film, wherein the film resulting by this method has a molecular weight distribution of about 4.2 or less and a die swell ratio of about 1.28 or more.
Further, the present invention provides a laminated molded body comprising a heat molded body which comprises a polypropylene resin composition containing 100 parts by weight of a polypropylene resin (A) having a die swell ratio of less than about 1.70 and about 0.01 to about 45 parts by weight of a polypropylene resin (B) having a die swell ratio of about 1.80 or more, and a substrate comprising a thermoplastic resin.